Identification of the human cytochrome P450 enzymes involved in the in vitro metabolism of artemisinin

Aims The study aimed to identify the specific human cytochrome P450 (CYP450 ) enzymes involved in the metabolism of artermisinin. Methods Microsomes from human B-lymphoblastoid cell lines transformed with individual CYP450 cDNAs were investigated for their capacity to metabolize artemisinin. The effect on artemisinin metabolism in human liver microsomes by chemical inhibitors selective for individual forms of CYP450 was invest igated. The relative contribution of individual CYP450 isoenzymes to artemi sinin metabolism in human liver microsomes was evaluated with a tree-based regression model of: artemisinin disappearance rate and specific CYP450 act ivities. Results The involvement of CYP2B6 in artermisinin metabolism was demonstrat ed by metabolism of artemisinin by recombinant CYP2B6, inhibition of artemi sinin disappearance in human liver microsomes by orphenadrine (76%) and pri mary inclusion of CYP2B6 in the tree-based regression model. Recombinant CY P3A4 was catalytically competent in metabolizing artemisinin, although the rate was 10% of that for recombinant CYP2B6. The tree-based regression mode l suggested CYP3A4 to be of importance in individuals with low CYP2B6 expre ssion. Even though ketoconazole inhibited artemisinin metabolism in human l iver microsomes by 46%, incubation with ketoconazole together with orphenad rine did not increase the inhibition of artemisinin metabolism compared to orphenadrine alone. Troleandomycin failed to inhibit artemisinin metabolism . The rate of artemisinin metabolism in recombinant CYP2A6 was 15% of that for recombinant CYP2B6. The inhibition of artemisinin metabolism in human l iver microsomes by 8-methoxypsoralen (a CYP2A6 inhibitor)was 82% but CYP2A6 activity was not included in the regression tree. Conclusions Artemisinin metabolism in human liver microsomes is mediated pr imarily by CYP2B6 with probable secondary contribution of CYP3A4 in individ uals with low CYP2B6 expression. The contribution of CYP2A6 to artemisinin metabolism is likely of minor importance.